In recent years both governmental and private agencies have stressed the damage which can result when eyes are exposed to bright sunlight, this damage being caused primarily by exposure to wavelengths in the ultraviolet portion of the radiation spectrum. Therefore, glass manufacturers have been active in designing compositions for sunglasses which will essentially block the transmittance of ultraviolet radiation. U.S. Pat. No. 5,256,607 (Kerko et al.) and U.S. Pat. No. 5,268,335 (Kerko et al.) provide two disclosures directed to the production of such glasses.
U.S. Pat. No. 5,256,607 describes glass compositions suitable for use as sunglasses which exhibit a neutral gray, fixed tint coloration and limit the transmittance of ultraviolet radiation at a wavelength of 380 nm to no more than 1% at a thickness of 2 mm. The glass compositions are preferably essentially free of ZnO and consist essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 65-72 K.sub.2 O:Na.sub.2 O 1.25-2.25 As.sub.2 O.sub.3 0-0.3 B.sub.2 O.sub.3 2-6 Al.sub.2 O.sub.3 0-2.25 Fe.sub.2 O.sub.3 4.8-6.2 Na.sub.2 O 6-10 CaO 0-1.5 Co.sub.3 O.sub.4 0.012-0.02 K.sub.2 O 10-16 Al.sub.2 O.sub.3 + 0-1.5 NiO 0.16-0.21. Na.sub.2 O + 17-23 CaO K.sub.2 O ______________________________________
The high concentration of iron provided the required absorption of ultraviolet radiation. Cobalt and nickel were added to adjust the chromaticity of the glass to a neutral gray target. Al.sub.2 O.sub.3, B.sub.2 O.sub.3, and/or CaO were included to assist in suppressing the transmittance of the glass at a wavelength of 380 nm to below 1% at a thickness of 2 mm.
U.S. Pat. No. 5,268,335 discloses the fabrication of ophthalmic lenses which can be chemically strengthened to high values with surface compression layers of substantial depth through short ion exchange treatments. Those glasses also restrict the transmittance of ultraviolet radiation at 380 nm to no more than 1% at a thickness of 2 mm. Iron was utilized to achieve the demanded low transmittance of ultraviolet absorption and, where a neutral gray coloration was desired, cobalt and nickel were included. The base compositions therefor were preferably free of ZnO and consisted essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 55-65 Na.sub.2 O 6-18 MgO 0-4 B.sub.2 O.sub.3 5-20 K.sub.2 O 2-10 ZrO.sub.2 0-7 Al.sub.2 O.sub.3 4-10 Li.sub.2 O + 13-22 MgO + 0-10 B.sub.2 O.sub.3 + 14-26 Na.sub.2 O + TiO.sub.2 + Al.sub.2 O.sub.3 K.sub.2 O ZrO.sub.2 Li.sub.2 O 0-3 CaO 0-1.5 As.sub.2 O.sub.3 0-0.5 MgO 0-4 ZnO 0-1.5. ______________________________________
Where other tints are desired in the glass, the patent cited the use of Co.sub.3 O.sub.4, Cr.sub.2 O.sub.3, CaO, Fe.sub.2 O.sub.3, MnO.sub.2, NiO, V.sub.2 O.sub.5, Au, Pd, and rare earth metal oxides.
The patent also disclosed a glass commercially marketed by Corning Incorporated, Corning, N.Y., which exhibits a neutral gray fixed tint and limits the transmittance of ultraviolet radiation at 380 nm to no more than 1% at a thickness of 2 mm. An analysis of the glass, marketed under Corning Code 8015, is tabulated below, expressed in terms of weight percent on the oxide basis:
______________________________________ SiO.sub.2 68.41 K.sub.2 O 9.71 Co.sub.3 O.sub.4 0.021 Al.sub.2 O.sub.3 0.51 ZnO 6.76 NiO 0.126 Na.sub.2 O 8.81 Fe.sub.2 O.sub.3 5.54 As.sub.2 O.sub.3 0.111. ______________________________________
There has been a need for a glass suitable for ophthalmic applications exhibiting a green fixed tint of a desirable hue that can be chemically strengthened to high values with surface compression layers of significant depth, and which limits the transmittance of ultraviolet radiation at 380 nm to less than 1% at a thickness of 2 mm.
Glasses displaying green colorations have been known for many years. In his monograph entitled "Coloured Glasses", Dawson's of Pall Mall, London, 1959, W. A. Weyl discusses the utility of the ions of chromium, cobalt, copper, vanadium, and praseodymium in producing green glasses. He also examined the palette of colors, including greenish hues, which result from the combination of Fe.sup.+2 and Fe.sup.+3 ions in the glass.
U.S. Pat. No. 2,937,952 (Smith et al.) discloses green glasses suitable for ophthalmic applications wherein the green coloration is imparted thereto through the inclusion of 2.8-5% Fe.sub.2 O.sub.3. The patent noted that, although the total iron was reported as Fe.sub.2 O.sub.3, the presence of both Fe.sup.+2 and Fe.sup.+3 was acknowledged. The glasses consisted essentially, in weight percent, of
______________________________________ SiO.sub.2 65-75 CaO 5-8 Na.sub.2 O 7-10 MgO 2-4 K.sub.2 O 7-10 CaO + MgO 7-10.5 Na.sub.2 O + K.sub.2 O 15.5-17.5 Fe.sub.2 O.sub.3 2.8-5. ______________________________________
U.S. Pat. No. 5,268,335, supra, explained that the incorporation of substantial quantities of CaO in a glass composition has customarily been observed to retard the ion exchange taking place between potassium ions from a salt bath and sodium ions present in the glass surface. That action of CaO leads to a shallower depth of the surface compression layer even after relatively long exchange periods such that, whereas providing a very substantial initial improvement in mechanical strength, considerable reduction in strength can result as a consequence of surface abuse suffered by the glass during use in service. The glasses described by Smith et al. contain substantial levels of CaO.
Corning Code 8053 glass, marketed by Corning Incorporated, exhibits a green hue and absorbs very strongly in the ultraviolet portion of the radiation spectrum. Both the color and the ultraviolet absorption are primarily the result of the high level of iron present in the glass, which glass consists essentially, as analyzed in weight percent as expressed on the oxide basis, of
______________________________________ SiO.sub.2 67.0 K.sub.2 O 13.1 B.sub.2 O.sub.3 7.5 ZnO 2.5 Al.sub.2 O.sub.3 2.0 TiO.sub.2 1.0 Na.sub.2 O 2.4 Fe.sub.2 O.sub.3 5.1. ______________________________________
Unfortunately, because the Na.sub.2 O content is low, the glass does not perform well when subjected to chemical strengthening in the standard commercial white crown glass salt bath; i.e., immersion for 2 to 16 hours in a bath of molten KNO.sub.3 operating at a temperature of 450.degree. C. Thus, there are insufficient Na.sup.+ ions in the glass surface to exchange with K.sup.+ ions from the salt bath so as to produce a surface compression layer of significant thickness.
Glass RB-3, marketed by Schott Glaswerk, Mainz, Germany, displays a green color and absorbs very strongly in the ultraviolet portion of the radiation spectrum. In like manner to Corning Code 8053 glass, supra, both the color and the ultraviolet absorption are due primarily to the high level of iron included in the glass composition, that composition consisting essentially, as analyzed in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 72.6 CaO 4.4 Al.sub.2 O.sub.3 0.54 As.sub.2 O.sub.3 0.12 Na.sub.2 O 10.6 Fe.sub.2 O.sub.3 5.76. K.sub.2 O 6.0 ______________________________________
That glass encounters problems in chemical strengthening on two counts. It contains a substantial amount of CaO and its strain point is below 450.degree. C. The retarding effect which CaO exerts on the ion exchange reaction comprising the basis of chemical strengthening was explained above. To cause a compression surface layer to develop, the strain point of the glass must be higher than the temperature of the salt bath; in this instance, higher than 450.degree. C. Otherwise, the bath temperature will cause the glass surface to relax such that compressive stresses are not built up therein.
Accordingly, the principal objective of the present invention was to devise glass compositions suitable for ophthalmic applications which exhibit a fixed green tint, which exhibit a transmittance to ultraviolet radiation at a wavelength of 380 nm of less than 1%, which will exhibit a transmittance having a dominant wavelength between 554-564 nm with a purity of 18-32%, which exhibit a refractive index of 1.523, and which can be chemically strengthened.